Aprilaire #4851 Blower Activation Relay. Allows digital control (#58) to turn on system fan when there is a call for humidification (but not heat).

For Aprilaire series 500, 600, and 700 humidifiers that use the Automatic Digital Humidifier Controls (# 56 and # 58). This device will initiate HVAC fan operation when there is a call for humidity, even when there is no heat call. Use this relay in applications where humidifier operation is required beyond a call for heat, e.g. when installed on a heat pump or during times of the year where there is a need for humidity but not a need for heat.

Note: Aprilaire recommends that humidifer is plumbed to hot water when using this relay.

Bypass humidifiers can put out a lot of humidity, but only if set up in the following way:1) hot water must be used (and insulate the copper piping all the way to humidifier solenoid)2) The humidifier must be able to work independently of the furnace/heater, ie activate the blower and itself when needed... and this part performs #2 perfectly.The humidity in my house in winter is 25-27%, and get dry itchy skin, dry eyes, nosebleeds. With humidifier only on when furnace is on, hovers around 30-32%. But after installing this part, humidifier can run long enough to keep humidity constantly at a comfortable 40-45% (needs to run about 75% of time to do so). I am using 1 600M model for 3200 sq feet, and I need about 9 gallons per day output to keep humidity above 40%.

wiring diagram is for new units with built in relay - will not work if wired as shown to older models with built in transformer; instead wire 4851 w/y and w/blue wires into solenoid, not humidifier wires. these wires need to see 24V only when hum is on - older units have 24V on hum wires when off!

This relay helps to humidify the house all the time, not just when the heating system has a call for heat. Any time the humidity in the home drops below a predetermined level the relay kicks on the humidifier and furnace blower. Humid air circulates through every room! The only downside is early in the heating season the humidifier doesn't seem to keep the air "moist" enough. Aprilaire claims this is common and has to do with the outdoor temp and your body becoming used to less humidity in the air than summer air.

I installed this on a gas system where the heat is constantly on/off/on/off and the humidifier doesn't run long enough. This works great because it will keep the fan going until desired humidity is reached.

Bypass humidifiers can put out a lot of humidity, but only if set up in the following way:1) hot water must be used (and insulate the copper piping all the way to humidifier solenoid)2) The humidifier must be able to work independently of the furnace/heater, ie activate the blower and itself when needed... and this part performs #2 perfectly.The humidity in my house in winter is 25-27%, and get dry itchy skin, dry eyes, nosebleeds. With humidifier only on when furnace is on, hovers around 30-32%. But after installing this part, humidifier can run long enough to keep humidity constantly at a comfortable 40-45% (needs to run about 75% of time to do so). I am using 1 600M model for 3200 sq feet, and I need about 9 gallons per day output to keep humidity above 40%.

wiring diagram is for new units with built in relay - will not work if wired as shown to older models with built in transformer; instead wire 4851 w/y and w/blue wires into solenoid, not humidifier wires. these wires need to see 24V only when hum is on - older units have 24V on hum wires when off!

This relay helps to humidify the house all the time, not just when the heating system has a call for heat. Any time the humidity in the home drops below a predetermined level the relay kicks on the humidifier and furnace blower. Humid air circulates through every room! The only downside is early in the heating season the humidifier doesn't seem to keep the air "moist" enough. Aprilaire claims this is common and has to do with the outdoor temp and your body becoming used to less humidity in the air than summer air.

I installed this on a gas system where the heat is constantly on/off/on/off and the humidifier doesn't run long enough. This works great because it will keep the fan going until desired humidity is reached.

Aprilaire #4851 Blower Activation Relay. Allows digital control (#58) to turn on system fan when there is a call for humidification (but not heat).

For Aprilaire series 500, 600, and 700 humidifiers that use the Automatic Digital Humidifier Controls (# 56 and # 58). This device will initiate HVAC fan operation when there is a call for humidity, even when there is no heat call. Use this relay in applications where humidifier operation is required beyond a call for heat, e.g. when installed on a heat pump or during times of the year where there is a need for humidity but not a need for heat.

Note: Aprilaire recommends that humidifer is plumbed to hot water when using this relay.